On a power conversion apparatus such as an inverter and the like, an insulated gate transistor such as IGBT or MOSFET is employed as a semiconductor switching element that controls a supply current flowing to a load. Moreover, the power conversion apparatus often integrates a current detecting element in which the current detecting element detects a current supplied to the load side through the insulated gate transistor to protect the insulated gate transistor and the like from suffering overcurrent.
At this point, the current detecting element is realized as a current-detecting insulated gate transistor (so-called a sense IGBT) 2 that is disposed in parallel to a main insulated gate transistor (main IGBT) 1 being an IGBT that controls a main current as shown in FIG. 3 and that outputs another current on a proportional basis in size between the transistors to the main current flowing through the main insulated gate transistor 1. Concretely, the current-detecting insulated gate transistor 2 is installed so that a collector electrode and a gate electrode thereof are connected to another collector electrode and another gate electrode of the main insulated gate transistor 1, respectively.
In this regard, the current-detecting insulated gate transistor 2 is entirely integrated in single-piece construction with the main insulated gate transistor 1 into a semiconductor substrate (not shown) such as Si or SiC, for example, resulting in materializing an insulated gate semiconductor device 33. In addition, the current-detecting insulated gate transistor 2 outputs the current being on a proportional basis in size between the transistors to the main current flowing through the main insulated gate transistor 1 from the emitter electrode thereof. And then, with regard to the insulated gate semiconductor device 33, there are provided external connection terminals connected to the collector electrode, the emitter electrode, and the gate electrode of the main insulated gate transistor 1, as well as the emitter electrode of the current-detecting insulated gate transistor 2, respectively.
Moreover, this kind of insulated gate semiconductor device 33 is configured so that a temperature detecting diode 4 is often installed to detect an operating temperature of the main insulated gate transistor 1. In addition, a Zener diode (ZD) 5 is generally connected in parallel with the temperature detecting diode 4 in order to prevent a malfunction caused by noises output from the temperature detecting diode 4 while regulating a voltage applied to the temperature detecting diode 4 and to compensate for temperature properties of the temperature detecting diode 4. The insulated gate semiconductor device 33 with this configuration is introduced in detail, for example, in Patent Literature 1 (see FIG. 2 of Patent Literature 1).
The insulated gate semiconductor device 33 is configured so that the main insulated gate transistor 1 is turned on and off by using a control circuit 14 comprising a driving circuit 11, a current detecting circuit 12, and a temperature detecting circuit 13. The driving circuit 11 applies a driving signal to the gate electrode of the main insulated gate transistor 1 via an output transistor 15, being, for example, a p-type MOSFET, and then turns on and off the main insulated gate transistor 1.
Furthermore, the current detecting circuit 12 monitors the main current flowing through the main insulated gate transistor 1 while detecting a voltage converted from the current output through the emitter electrode of the current-detecting insulated gate transistor 2 via a current detecting resistance (Rs) 16. Then, the current detecting circuit 12 controls the driving circuit 11 to suspend, turning off the main insulated gate transistor 1. This prevents the main insulated gate transistor 1 from suffering overcurrent disruption owing to an excessively high current.
In addition, the temperature detecting circuit 13 detects an operating temperature of the insulated gate semiconductor device 33 based on still another current flowing through the temperature detecting diode 4 and another operating temperature of the insulated gate transistor 1 consequently. Then the control circuit 14 controls the driving circuit 11 to suspend, turning off the main insulated gate transistor 1 when an abnormal temperature increase is detected by the temperature detecting circuit 13. This prevents the main insulated gate transistor 1 from suffering overheat disruption.
In this regard, on the insulated gate semiconductor device 33 described above, there is a problem that electro-static discharge (ESD), which is determined based on the Machine Model (MM) or the Human Body Model (HBM), may produce dielectric breakdown to a gate oxide and an interlayer dielectric film in the insulated gate semiconductor device. The current-detecting insulated gate transistor 2 is designed as an element feeding about one-several hundredth to one-ten thousandth of current on a proportional basis to the main current flowing in the main insulated gate transistor 1. Then an area of the current-detecting insulated gate transistor 2, as viewed from a cross-section of the transistor 2, is approximately one-several hundredth to one-ten thousandth of the main insulated gate transistor 1. And then a value of parasitic capacitance created in the gate oxide or the interlayer dielectric film is on a proportional basis in size between the insulated gate transistors. As a result, ESD tolerance decreases remarkably in the current-detecting insulated gate transistor 2 when compared with that of the main insulated gate transistor 1.
On the other hand, a technical method is known as a means for improving ESD tolerance so that a diode for protecting electro-static discharge is interposed as described in Patent Literature 2 to protect the current-detecting insulated gate transistor 2 without changing output characteristics thereof by dimensions of a junction area with respect to a pn junction as well as by a static breakdown voltage of the diode (see FIG. 1 in Patent Literature 2).